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MAX712 Charger

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Connor

New Member
Hey Folks,

I'm looking to build a charger to charge a 14.4v 3800mAh pack. Well, I really want to charge 4 of these up. But, from what I can tell, charging NiMH packs in parallel is bad.. So, I though I could build 4 charging circuits and wire them in parallel using Diodes. I've read the Data sheet on the MAX712, and It looks like what I need. I want to be able to charge these packs at around 1.25 to 1.5Amps. Source voltage will be a 18v power supply with up to 8amps output. The circuit needs to be able to handle the load while charging.. The load can peak at around 1.7Amps. I thought about using larger batteries such as D's or F's.. However, charging multiple packs at the same time would yield a faster charge.. and the most mAh I can get with other cell's is either 10000mAh with D's or 14000mAh with the F's. 4 3800mAh packs gets me longer runtime. Any help would be appreciated!

Thanks, Connor
 

audioguru

Well-Known Member
Most Helpful Member
The datasheet for the MAX712 shows each cell's voltage exceeding 1.5V. The datasheet says that the input voltage must be at least 1.5V higher than the max battery voltage. You are charging 12 cells so you need an input voltage at least 20.1V.

The IC has only a single input to detect when the battery is fully charged. Then you can't add more batteries with or without diodes.
 

Connor

New Member
audioguru said:
The datasheet for the MAX712 shows each cell's voltage exceeding 1.5V. The datasheet says that the input voltage must be at least 1.5V higher than the max battery voltage. You are charging 12 cells so you need an input voltage at least 20.1V.
The power supply I'm using is a universal power notebook adapter. Output is rated for 15-24v up to 8.0A 120W Max. Current plug I'm using is around 18.5v or so.. The battery packs are 14.4v packs. So, I thought I would be good with 18v??

audioguru said:
The IC has only a single input to detect when the battery is fully charged. Then you can't add more batteries with or without diodes.
I was thinking about using 4 MAX712's 1 for each pack, then put diodes on the output side of the circuit before wiring them to the load, Thus, each MAX712 is responsible for it's own pack.. and the load is shared across all 4 packs/chargers.

Thanks, Connor
 

audioguru

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If the input voltage is not at least 21.1V then your batteries will not fully charge.
Don't use diodes at the output of each MAX712. Each one charges its own battery. They cannot share a load because each one must detect that its battery is fully charged.
 

Connor

New Member
audioguru said:
Don't use diodes at the output of each MAX712. Each one charges its own battery. They cannot share a load because each one must detect that its battery is fully charged.
This is the part I don't understand. The MAX712 says it can charge a battery while supplying a load. Looking at the found @ http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1666 You couldn't put a diodes right before the load so that the charging circuits were isolated as far as the charging side?

Thanks, Connor
 

T47

New Member
I want to use this ic for a project im building, but after reading the datasheet i found the part about the number of cells programed a little confusing. it says: "The internal ADC’s input voltage range is limited to between 1.4V an 1.9V, and is equal to the voltage across the battery divided by the number of
cells programmed". does this mean i cant use it with a single 8.4V 200mAh NiMH battery? Would using 8x 1.2V 1400 mAh (AA) batterys be better? And finaly if i do use the AA batterys in series will the current/h rating be additive (iow 8x 1400mAh = 11,2 Ah)?
 

audioguru

Well-Known Member
Most Helpful Member
The Ah rating of a battery is for its current. Cells in series all have the same current as one cell. So the Ah rating of a battery with one cell is the same Ah rating of a series srting of many cells.

I didn't read the entire datasheet. You must program how many cells you have and I think a resistor determines the charging current.
 

zero9

New Member
Hey!
i just found this post in a google search. I want to build a circuit using this MAX 712 chip. I understand that when the device is in charge mode, the load is supplied by the DCIn. My question is: when the device is in trickle mode, which power source supplies the load? If the DC In is connected, and the battery is fully charged, does the battery or the DCIN supply the load?
 

audioguru

Well-Known Member
Most Helpful Member
The load is always connected directly to the power source while the battery charger has its own low side feed of current through the current sense resistor.

When the power source is gone then the battery powers the load through the current sense resistor.
 

unseen

New Member
Hi,

I have built the charger circuit based on the MAX712/MAX713 from this site: Fast, smart and universal NiCd/NiMh battery charger

I've tried with both used and brand new NiMh batteries of 1100 and 1300mAh capacity.
The circuit seems to work fine, except that it SWITCHES OFF after about 15-20 minutes of charging!
(After no more than 5 pictuers, my camera turns off saying that batteries are depleted)
Set to 4 hours max charge time according to datasheet of MAX712. (PGM2, PGM3 = BATT-)
I also tried from 1 cell to 4 cells, (+ dip switches changed) Rsense 0.6 and 1 Ohm, Rshunt 0,6K and 1,4K Ohms with same results.
Charging currents are fine. (250mA / 1 Ohm, 415mA / 0,6 Ohm)

What could be wrong? What should I check?

Could be that I am using a BD 434 on a decent heat sink? By touch it's slightly warm, I'd say around 45-50 deg C, so it handles that current easily.

Thanks and Happy New Year!
 
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